In recent years, with progress in the digital technology, analog signals such as audio or video signals are sampled, quantized and encoded for transmitting, recording and/or reproducing as what is called pulse code modulated (PCM) signal.
In converting an analog signal into a PCM digital signal for transmission, the higher the sampling frequency, the wider the range of the transmittable analog signal and, the larger the number of quantization bits, the wider the dynamic range, all in a well-known manner. Therefore, when the original analog signal is to be transmitted with high fidelity, that is with a wide frequency band and a large dynamic range, a high sampling frequency and a large number of quantization bits are required, with the result that the number of bits to be transmitted per unit time or bit rate is increased.
However, the bit rate is limited by the properties of the transmitting medium or recording medium and the digital signal processing speed at the transmitting and/or receiving sides or at the recording and/or reproducing sides. As a matter of fact, in consideration of economy or cost performance in supplying the apparatus such as PCM signal recording and/or reproducing apparatus, it becomes critical to transmit, record or reproduce a high-quality signal at as low a bit rate as possible.
For transmitting large dynamic range signals with a lower bit rate, there is known in the art a digital signal processing system making use of linear prediction, such as differential PCM system. For example, in a system proposed by the present inventors, the digital signal data to be transmitted are grouped into blocks each comprised of a predetermined number of words and transmitted through one of a plurality of prediction filters. Three different kinds of the prediction filters are provided for outputting straight PCM data, differential PCM data and additive PCM data. The prediction filters or the PCM modes that will give the maximum data compression efficiency as a function of an input signal are selected every block. Selection of the PCM modes is by detecting the maximum absolute value of the straight PCM data in one block, the maximum absolute value of the differential PCM data in one block and the maximum additive PCM data in one block, and comparing the three maximum absolute values in the respective blocks to one another to find the PCM mode that will give the least value.
With a small number of the prediction filters or of the selectable PCM modes, such as three, the optimum prediction filter or PCM mode can be selected by computing the data of the respective PCM modes and directly comparing these various data to one another. However, when the number of the prediction filters is increased for further elevating the bit compression efficiency, or when the prediction filters are designed so that plural stages, of the prediction coefficients are selectable it becomes necessary to compute all filter output data of the selectable modes and hence to carry out high speed or parallel processing. This means a considerable load in hardware.
It is an object of the present invention to provide an apparatus for transmitting digital signals, in which, even when a larger number of the prediction filters or a larger number of types of the prediction filters are provided, it is possible to select an optimum one of a large number of the prediction filters including the selection filters as a function of the input signal by means of a simple construction employing selection filters of a number sufficiently smaller than that of the prediction filters, so that elevation of hardware costa may be avoided.